RAINS AT ST. HELENA. 85 



rain-producing factors of the world, yet, " in the region 

 of the trades the rainfall is everywhere small over the 

 open sea, " * i st, " because their course is directed into 

 regions that become constantly warmer " f and 2ndly, 

 because their steady nature leaves few opportunities for 

 meeting with variable currents of colder air. Thus at 

 Ascension, in Latitude 8 45' S., which is always within 

 the course of the S.E. trades, the mean rainfall for 

 1854 and '55 was only 8.85 inches, and at St. Helena, 

 also situated within the same, the observations of five 

 years give a rainfall of only 5.36 inches on the coast. 

 But the moment the interior of that island is reached, 

 observations taken at an altitude of 1763 feet above 

 sea-level, according to the same authority, show -that 

 it has immediately increased to 23.98 inches, because 

 of the fall in the temperature produced by this increased 

 elevation. We have drawn particular attention to 

 these facts, as showing how completely rainfall is 

 governed by variations of temperature. 



The influence of the trade winds upon navigation 

 has, Lieut. Maury tells us, been laboriously investi- 

 gated at the National Observatory at Washington, and 

 results show that the average speed of vessels sailing 

 through the south east trades of the Atlantic, which 

 are both stronger and steadier than the N.E., was 6.1 

 knots an hour between the parallels 5 and 10 degrees 

 South; and 5.7 knots between 25 and 30 degrees 

 South. ** 



Experience shows that sailing ships, after entering 



* Encycl. Brit., gth edit. Vol. xvi, p. 150. 



f Ibid., p. 150. 



Ibid., p. 150. 



** Lieut. Maury, U.S.N., "Sailing Directions," 8th Edit. Vol. ii, p. 857. 



